The invention relates to a device for providing glass layers on the inside of a tube, comprising a gas supply device which is connected to one end of the tube, a furnace for heating the tube, a resonator comprising a resonant cavity for generating a plasma in the tube, and means to move the resonator and the tube relatively with respect to each other, a high-frequency generator which is connected to the resonant cavity and a vacuum pump which is connected to the opposite end of the glass tube.
Such a device is disclosed, for example, in European Patent Application No. 0129129. In this known device means are present to rotate the tube during the deposition according to a given scheme. Devices of this type are used to manufacture preforms for drawing optical fibres.
One method to increase the deposition rate is to increase the power supplied to the plasma and the quantity of gases supplied to the tube per unit of time for the formation of glass layers. However, the heat radiation per surface unit by the plasma also increases. An imperative condition for the perfect performance of the so-called non-isothermal plasma CVD process for the manufacture of optical fibres is that the temperature in the space enclosed by the tube is maintained at a temperature below the temperature at which the homogeneous reaction occurs in the gas in the tube. In practice the temperature of the tube is maintained below 1200.degree. C. If a homogeneous reaction occurs, glass soot particles are formed in the gaseous phase and are incorporated in the glass layers which are formed simultaneously by a heterogeneous reaction. Then a clear glass layer is not obtained. Another problem, which is playing a more and more important role, is that upon increasing the power dissipated in the plasma and the greater heat radiation per unit of surface by the plasma associated therewith is the non-uniform heating of the tube by the plasma over the length of the tube and in time. That such a non-uniform heating of the tube by the plasma is bound to occur can easily be explained.
Between the reversal points of the plasma in the tube there is a point where the plasma passes after equal periods of time. On the left and on the right of this point time differences occur between successive passages of the plasma. Furthermore, at the reversal points of the movement of the plasma a velocity delay occurs as a result of which the relative residence of the plasma and hence the heat radiation per surface unit is increased at that area. A further complication is that the plasma is not present symmetrically within the resonant cavity with respect thereto but generally is shifted with respect to the resonant cavity in the direction of the vacuum pump. Moreover, the heat radiation per surface unit by the plasma over the length of the plasma is not equal.